This disclosure relates generally to the field of seismic evaluation of formations in the Earth's subsurface. More specifically, the invention relates to methods for determining seismic attributes in specific formations such as non-linearities in formation response to acoustic energy.
U.S. Pat. No. 7,830,478 issued to Guigné et al. describes a method for seismic surveying includes disposing a plurality of seismic sensors in a selected pattern above an area of the Earth's subsurface to be evaluated. A seismic energy source is repeatedly actuated proximate the seismic sensors. Signals generated by the seismic sensors, indexed in time with respect to each actuation of the seismic energy source are recorded. The recorded signals are processed to generate an image corresponding to at least one point in the subsurface. The processing includes stacking recordings from each sensor for a plurality of actuations of the source and beam steering a response of the seismic sensors such that the at least one point is equivalent to a focal point of a response of the plurality of sensors.
It is known in the art that micro-inhomogeneous regions of rock can have very large non linear effect on seismic signals, described by a large value of the nonlinearity parameter β. The term nonlinear in the context of vibroseis often refers to the nature of the amplitude and frequency of the drive to the plate being a nonlinear function of time. In this disclosure the term “ramped chirps” is used to indicated the chirp amplitude is not constant with time. The value of β for water is about 3 and it is claimed that micro-inhomogeneous regions of rock yield effective values of β that may be larger than those of isotropic solids by several orders of magnitude. Values of β between 80 and 1000 have been observed for rocks.
There exists a need for acoustic evaluation of the subsurface that enables identification of regions of non-linearities.